Problem: Starting at home, Luis traveled uphill to the hardware store for 20 minutes at just 12 mph. He then traveled back home along the same path downhill at a speed of 24 mph. What is his average speed for the entire trip from home to the hardware store and back?
Answer: The average speed is not just the average of 12 mph and 24 mph. He traveled for a longer time uphill (since he was going slower), so we can estimate that the average speed is closer to 12 mph than 24 mph. To calculate the average speed, we will make use of the following: $\text{average speed} = \dfrac{{\text{total distance}}}{{\text{total time}}}$ $\text{distance uphill} = \text{distance downhill}$ What was the total distance traveled? ${\begin{align*}\text{total distance} &= \text{distance uphill} + \text{distance downhill}\\ &= 2 \times \text{distance uphill}\end{align*}}$ $\begin{align*}\text{distance uphill} &= \text{speed uphill} \times \text{time uphill} \\\ &= 12\text{ mph} \times 20\text{ minutes}\times\dfrac{1 \text{ hour}}{60 \text{ minutes}}\\ &= 4\text{ miles}\end{align*}$ Substituting to find the total distance: ${\text{total distance} = 8\text{ miles}}$ What was the total time spent traveling? ${\text{total time} = \text{time uphill} + \text{time downhill}}$ $\begin{align*}\text{time downhill} &= \dfrac{\text{distance downhill}}{\text{speed downhill}}\\ &= \dfrac{4\text{ miles}}{24\text{ mph}}\times\dfrac{60 \text{ minutes}}{1 \text{ hour}}\\ &= 10\text{ minutes}\end{align*}$ ${\begin{align*}\text{total time} &= 20\text{ minutes} + 10\text{ minutes}\\ &= 30\text{ minutes}\end{align*}}$ Now that we know both the total distance and total time, we can find the average speed. $\begin{align*}\text{average speed} &= \dfrac{{\text{total distance}}}{{\text{total time}}}\\ &= \dfrac{{8\text{ miles}}}{{30\text{ minutes}}}\times\dfrac{60 \text{ minutes}}{1 \text{ hour}}\\ &= 16\text{ mph}\end{align*}$ The average speed is 16 mph, and which is closer to 12 mph than 24 mph as we expected.